1. Field of the Invention
The present invention relates to a photoelectric conversion device, a method of manufacturing the photoelectric conversion device and, more particularly, to a CMOS area sensor, a method of manufacturing the CMOS area sensor, and an image pickup system using the CMOS sensor.
2. Related Background Art
Charge-coupled devices (CCD) are known as a solid-state image pickup device which converts an image signal into an electric signal. CCDs have a photodiode array in which a pulse voltage is applied to electric charges accumulated in photodiodes to read out the charges as an electrical signal. In recent years, complementary metal-oxide-semiconductor (CMOS) area sensors having photodiodes and peripheral circuits including MOS transistors into one chip have been used as a solid-state image pickup device. CMOS area sensors have the advantages of having a lower power consumption and a lower drive power in comparison with CCDs and the demand for them is expected to increase in future.
A CMOS area sensor will be described with reference to FIG. 9 as a typical example of a photoelectric conversion device. FIG. 9 is a schematic cross-sectional view of a photodiode portion 301 and a transfer MOS transistor portion 302 of the CMOS area sensor. FIG. 9 shows an n-type silicon substrate 303, a p-type well 304, a gate electrode 307 of the transfer MOS transistor, an n-type charge accumulation region 308 of the photodiode, a surface p-type region 309 of the photodiode for forming the photodiode in a buried structure, a field oxide film 305 for element separation, an n-type high-concentration region 310 in which floating diffusion is formed and which functions as a drain region of the transfer MOS transistor, a silicon oxide film 311 for insulation between the gate electrode and a first wiring layer 313, a contact plug 312, the first wiring layer 313, an interlayer insulating film 314 for insulation between the first wiring layer 313 and a second wiring layer 315, the second wiring layer 315, an interlayer insulating film 316 for insulation between the second wiring layer 315 and a third wiring layer 317, the third wiring layer 317, and a passivation film 318. A color filter layer (not shown) is formed on the passivation film 318, and a microlens (not shown) is formed on the color filter layer for an improvement in sensitivity. Light incident on the surface enters the photodiode through an opening defined in the third wiring layer 317. The light is absorbed in the n-type charge accumulation region 308 or the p-type well region 304 to generate electron-hole pairs. The electrons in the electron-hole pairs are accumulated in the n-type charge accumulation region 308.
As a CMOS area sensor structure according to a conventional art, a structure having a carrier profile such as shown in FIG. 10 (FIG. 6 of U.S. Pat. No. 6,483,129) is known. This structure has a high-concentration impurity diffusion region 6A in a deep region in a substrate and is thought to have the effect of improving the sensitivity by increasing the efficiency with which electric charge generated by absorption of light in a well is extracted to the surface side.
In conventional photoelectric conversion devices, particularly CMOS area sensors, a concentration distribution is formed so that the concentration decreases gradually in the substrate depth direction, as shown in FIG. 11, because the well layer of the photodiode is formed by performing thermal diffusion after ion implantation. As a result, a structure having no potential barrier in the substrate depth direction is formed, and part of light absorbed in the p-type well is lost in the direction of the substrate and does not contribute to the photoelectrically converted signal. In particular, a problem that the necessary sensitivity cannot be obtained has arisen with the reduction in pixel size. Also, there are few parameters of manufacturing conditions which can be handled at the time of control of characteristics such as the sensitivity, the number of saturated charges and characteristics of transfer from the photodiode to floating diffusion. Therefore there is also a problem that the performance requirements relating to such characteristics cannot be satisfied.
On the other hand, the structure shown in FIG. 10 of the above-mentioned patent document has a high-concentration impurity diffusion region in a deep region of a substrate and is thought to be effective in improving the sensitivity. However, the number of parameters of manufacturing conditions which can be handled at the time of control of various characteristics, including the number of saturated charges and characteristics of transfer from the photodiode to floating diffusion, which are to be simultaneously satisfied, is small and the performance requirements relating to such characteristics cannot be satisfied. In the case of a simple retrograde well structure such as that described in the above-mentioned U.S. Pat. No. 6,483,129, a dark current generated in the substrate leaks and enters the photodiode to cause degradation of the performance of the sensor. That is, no technical theme has been found to simultaneously achieve an improvement in sensitivity, an increase in the number of saturated charges and an improvement in transfer efficiency.